(I) Conventionally, bisphenol-based epoxy resins have been widely used in the fields of paints, electrical art, civil engineering and construction, adhesives and the like, because of their excellent adhesive power, resistance to chemicals, electrical characteristics and toughness. Especially, with respect to the use for cationic electrophoretic coating compositions, powder coating compositions, solvent-based epoxy resin coating compositions, etc., bisphenol A-based high molecular weight epoxy resins having a molecular weight of 900 to 2000 are used. However, such bisphenol-based epoxy resins are poor in weatherability because they have benzene rings as contained in the skeleton.
As high molecular weight epoxy resins containing no benzene ring, known are those prepared by converting an alicyclic epoxy compound such as diglycidyl ether of hydrogenated bisphenol A or 3,4'-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate to a high molecular weight resin by means of a polycarboxylic acid. However, these epoxy resins contain ester linkages and therefore are susceptible to hydrolysis and unsatisfactory in weatherability.
As described above, the epoxy resins as materials for said various uses remain to be improved, but high molecular weight epoxy resins containing no benzene ring and ester linkage are not known so far.
(II) Epoxy resins also have been conventionally employed in combination with a cationic photoinitiator for use as a photo-curable resin composition.
Conventionally, acrylic resins are known as photo-curable resins but reportedly have the problems that, since said resin cures by radical polymerization reaction, the polymerization thereof is partly inhibited due to oxygen in the air and they are unsatisfactory in the surface hardening properties, and that acrylic monomers are highly toxic and that said acrylic resin is low in adhesion to metals and plastics.
In recent years, attention is drawn to photo-curable resin compositions comprising a cationically polymerizable substance and a cationic photoinitiator because surface hardening thereof is not hindered by the air and because said compositions have low toxicity, less skin irritation, high adhesion to metals and plastics, etc. Such resin compositions have been already used for coating cans and for coating plastics.
Examples of the cationically polymerizable substances are bisphenol A-based epoxy resins, phenol novolak-based epoxy resins, cresol novolak-based epoxy resins, aliphatic glycidyl ether compounds, alicyclic epoxy resins such as 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexanecarboxylate, di(3,4-epoxycyclohexylmethyl)adipate, etc. which are disclosed, e.g. in Japanese Examined Patent Publication (Kokoku) No. 14278/1977 and Japanese Unexamined Patent Publication (Kokai) No. 239402/1993, and vinyl monomers, cyclic ethers, vinyl ethers and other oxirane oxygen-free monomers or their oligomers which are disclosed in Japanese Unexamined Patent Publication No. 32831/1978.
However, said cationically polymerizable substances can not be suited for all applications. For example, aromatic moiety-containing epoxy resins such as bisphenol A-based epoxy resins, phenol novolak-based epoxy resins, cresol novolak-based epoxy resins, etc. have low photo-curing rate, and furthermore the cured product obtained can not be used as coating materials for outdoor applications because of low weatherability. Moreover, it is difficult to photo-cure a thick film of these resins.
Alicyclic epoxy resins such as 3,4-epoxycyclohexylmethyl-3'4'-epoxycyclohexanecarboxylate, di(3,4-epoxycyclohexylmethyl)adipate, etc. have high level of photo-curing rate sufficient for commercial use, and are excellent in weatherability. However, there is a limitation in the commercial availability of these resins, and it is difficult to uniformly cure a thick film thereof. Furthermore, these resins have the drawback of giving coatings which are brittle and low in flexibility, impact resistance and adhesion.
Vinyl ethers have high photo-curing rate, but emit an offensive odor and, when added in a large amount, give coatings which are poor in hardness, flexibility, impact resistance, adhesion, thermal coloring resistance and other properties. The conjoint use of the cationic polymerizable substances can alleviate the drawbacks to some extent but can not be a complete countermeasure at present. These problems are a great obstacle to wider applications of cationic photopolymerizable resins.
Additionally, basic components of the photo-curable resin compositions comprise a cationic polymerizable substance and a cationic photoinitiator, and it is desired to combine the polymerizable substance with the cationic photoinitiator which is excellent in compatibility with the cationic polymerizable substance, in thermal coloring resistance and in weatherability.
Thus, cationic polymerizable substances which are outstanding in weatherability, high in photo-curing rate, uniformly curable throughout a thick coating film and excellent in film properties such as adhesion, flexibility, impact resistance and the like are not known. Also unknown are photo-curable resin compositions containing a cationic photoinitiator which is superior in the compatibility, thermal coloring resistance and weatherability.
(III) Epoxy resins have been used in the field of powder coatings as well.
Generally, powder coatings have the advantages of being excellent in film properties, enabling recovery for reuse, ensuring a high coating efficiency, facilitating automation, etc., and, from a viewpoint of environmental conservation, they have a great advantage of being low-pollutant due to the absence of a solvent.
Mainly used as powder coatings are epoxy-based powder coatings comprising a high molecular weight epoxy resin and a curing agent such as acid anhydrides, amine derivatives, phenolic resins, etc., hybrid powder coatings comprising a carboxyl group-containing polyester resin and a high molecular weight epoxy resin, polyester/TGIC-based powder coatings comprising a carboxyl group-containing polyester resin and triglycidyl isocyanurate (hereinafter referred to as "TGIC"), and polyester/urethane resin-based powder coatings comprising a hydroxyl group-containing polyester resin and a blocked polyisocyanate as a curing agent.
Conventionally, epoxy-based and hybrid powder coatings predominantly contain a high molecular weight bisphenol A type glycidyl ether as the epoxy resin component, and thus have the drawback of having low weatherability. For applications which particularly require weatherability, polyester/TGIC-based and polyester/urethane resin-based powder coatings have been used.
Polyester resins frequently used as the main component of powder coatings are prepared from a polyhydric alcohol and a polycarboxylic acid as the starting materials. A wide variety of polyester resins different in formulation, degree of polymerization, melting point, etc. can be produced by varying the combination of these starting materials, and a proper combination thereof can produce a powder coating well balanced in costs, pigment dispersibility, weatherability, flexibility and adhesion of the coating film.
Polyester/urethane resin-based powder coatings are excellent in film properties such as coating appearance, weatherability, corrosion resistance, etc.
However, these powder coatings contain, as a curing agent, a polyisocyanate whose active sites are blocked with caprolactam or the like, and thus must be cured upon removal of the blocking agent at a high temperature during baking, and at the same time pose the problem of environmental pollution arising from the evaporation of the blocking agent.
On the other hand, polyester/TGIC-based powder coatings essentially do not involve the evaporation of volatile component during baking, are curable at a low temperature and give cured coating films having good appearance and high weatherability, corrosion resistance, chemical resistance, etc. Consequently, these powder coatings have characteristics suitable for coping with environmental problems. However, TGIC has an average molecular weight of as low as about 300 and is inevitably toxic to humans and objectionable for skin irritation, mutagenicity, etc. The toxicity of TGIC used for powder coatings is a problem, and an epoxy compound to be used in place of TGIC should be one having a high molecular weight.
As set forth above, resin compositions for powder coatings which are excellent in film properties and weatherability, free of the evaporation of volatile component during baking and low in toxicity are not known. Consequently, there is a demand for resin compositions for powder coatings which are usable as a substitute for the polyester/TGIC-based powder coatings and which have high weatherability.